Hydrogen and a higher boiling compound are often used as synthesis gas components for the formation of useful chemical compounds. For example, synthesis gas having hydrogen and nitrogen components in a 3:1 molar ratio is employed to form ammonia. Another example is synthesis gas having hydrogen and carbon monoxide components in a 2:1 ratio which is used to form methanol. Still another example is synthesis gas having hydrogen and carbon monoxide components in a 1:1 ratio which is employed in the oxo process to form C.sub.4 and heavier alcohols and aldehydes.
The synthesis gas mixture is in most cases generated by steam reforming of hydrocarbons, in particular natural gas. Methane conversion in the steam reforming process is incomplete, so that low percentage levels of methane remain as an impurity in the synthesis gas. Generation of ammonia synthesis gas employs, in addition, a secondary reforming step, wherein air is introduced in a controlled manner to supply the necessary amount of nitrogen. The oxygen is consumed by partial oxidation of residual hydrocarbons. yielding additional synthesis gas. The argon content of the air remains, however. as an impurity in the ammonia synthesis gas.
In the ammonia synthesis process, fresh synthesis gas is added to a recycle loop which recirculates through the ammonia synthesis reactor. Unreacted components from the product side of the reactor are separated from the ammonia product, which is condensed by cooling the stream, and recycled through the reactor together with the fresh synthesis gas. Inert components, specifically methane and argon, build-up in concentration within this loop. In order to limit this build-up, a continuous purge fraction is normally withdrawn from the recycle loop, thus removing an absolute quantity of the inert components which equals that which has been brought into the loop by the fresh synthesis gas. It is often desirable to recover the unreacted synthesis gas components from this purge stream for recycle or other use. It may also be desirable to recover the impurities as additional products from the purge gas stream.
The methanol or oxo processes are frequently used in conjunction with other processes which require a relatively pure source of either hydrogen or carbon monoxide. It is often desirable to generate sufficient synthesis gas of the correct overall composition to satisfy the total hydrogen and carbon monoxide requirements of the facility. A portion of the gas may then be processed to recover pure component products. The hydrogen to carbon monoxide ratio(s) in the remaining synthesis gas fraction(s) may then be adjusted, as necessary, by appropriate blending of the various streams.
Cryogenic processing is one means which has heretofore been employed for the recovery of components from a synthesis gas stream. The cryogenic process employs a first partial condensation step which separates a hydrogen-rich vapor fraction from the remainder of the stream. The purity of the hydrogen which can be produced by this step is limited, since the low temperatures which would be required for complete condensation of the higher boiling synthesis gas component would result in freezing of the condensed fraction. The hydrogen-rich vapor might be directly utilized for recycle to the synthesis gas process. However, if high purity hydrogen is required, an additional processing method must be employed. Pressure swing adsorption is an example of such a method.
The condensate which is produced by the partial condensation step may be further processed in one or more cryogenic distillation columns in order to separate the higher boiling synthesis gas component and the heavier impurity fraction(s). However, the conventional recovery methods cannot recover the higher boiling synthesis gas component without a significant hydrogen presence in the stream. This is because hydrogen has significant solubility in the condensate which is formed in the presence of the hydrogen-rich vapor product. While this hydrogen contamination is not a problem when the higher boiling synthesis gas component is recycled for synthesis gas reaction, it may be a problem if it is desired to employ the recovered higher boiling synthesis gas component in an application which requires very high purity, and especially in an application which is sensitive to the presence of hydrogen.
Accordingly, it is an object of this invention to provide a process to recover, from a hydrogen-containing synthesis gas stream, a higher boiling synthesis gas component substantially free of hydrogen.